Selecting rows where a field is null using PHP PDO prepared statements and MySQL

Question

I've been converting an app to use PDO prepared statements rather than mysqli and I'm running into a strange problem. I have some records in the database where it's expected that a field will be null. Not 'null' (string), or '' (empty string), but NULL. I build my queries dynamically, so in the past when I came across a null variable in an object, I'd build the query like this:

WHERE fieldName is null;

And would get the expected results when the field was null.

Now with PDO, my queries aren't returning any results and I'm not getting any errors. It just simply isn't returning the records I would expect. When I echo the built queries and run them directly in MySQL I get the expected results, but within the application there are no results returned.

Some of the things I've tried include building queries that look like this:

WHERE fieldName is null;

or

WHERE fieldName <=> null;

I have also tried the standard prepared statement of:

WHERE fieldName = :fieldName

and then binding with these kinds of statements:

$stmt->bindParam(":$field", $value);
$stmt->bindParam(":$field", $value, PDO::PARAM_NULL);
$stmt->bindParam(":$field", null, PDO::PARAM_NULL);
$stmt->bindValue(":$field", null, PDO::PARAM_NULL);
$stmt->bindValue(":$field", null, PDO::PARAM_INT);

Any help with this would be greatly appreciated. My PHP version is 5.3.10 and MySQL is 5.5.22. As a side question, I still am not clear on the difference between bindParam and bindValue, so if it makes sense to include in your answer I would really appreciate some clarification on the subject...

Solution

Since this question has been written, mysql introduced a spaceship operator that allows us to use a regular query to match a null value

WHERE fieldName <=> :fieldName;

will match both a null or any not null value.

So just write your query right away and execute it as usual

$stmt = $db->prepare('SELECT field FROM table WHERE fieldName <=> :fieldName;');
$stmt->execute(['fieldName' => null]);
$result = $stmt->fetchAll(); // whatever fetch method is suitable

And with dynamically built queries it's all the same.

Comments

[Seth McCauley]

I did not think it was possible to use placeholders for an IS condition in a PDO statement, although I am having difficultly finding definitive proof of this. In testing, I cannot get "WHERE fieldName is :fieldName" to work, even using the exact code you provided. I can get that code to work fine if I use something other than IS, such as = or >. Can you elaborate on the code you used, or have any other suggestions? I'm stuck with MySQL version 5.1 though, so that may explain why it doesn't work for me (I see that you're using 5.5).

[Timo Huovinen]

WHERE fieldName = :fieldName will work for "bob" but not for when :fieldName is "NULL", where WHERE fieldName IS :fieldName will work for "NULL" but not for when :fieldName is "bob",